of hydrogen gas, generally a result of being recharged. The cell covers typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that may be formed during overcharge. They are not permanently sealed, but are designated to be maintenance free. They can be oriented in any manner, unlike normal lead–acid batteries, which must be kept upright to avoid acid spills and to keep the plates' orientation vertical. Cells may be operated with the plates horizontal (pancake style), which may improve cycle life. Absorbent glass mat (AGM) AGM batteries differ from flooded lead–acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. Very thin glass fibers are woven into a mat to increase the surface area enough to hold a sufficient amount of electrolyte on the cells for their lifetime. The fibers that compose the fine glass mat do not absorb and are not affected by the acidic electrolyte. These mats are wrung out 2–5% after being soaked in acids just prior to finish manufacturing. The plates in an AGM battery may be of any shape. Some are flat, whereas others are bent or rolled. Both deep cycle and starting type of AGM batteries, are built into a rectangular case according to Battery Council International (BCI) battery code specifications. AGM batteries are more resistant to self discharging than conventional batteries within a wide range of temperatures. As with lead–acid batteries, in order to maximize the life of an AGM battery, it is important to follow the manufacturer's charging specifications and the use of a voltage regulated charger is recommended. There is a direct correlation between the depth of discharge (DOD) and the cycle life of the battery, with differences between 500 and 1300 cycles depending on DOD. Gel battery Originally a kind of gel battery was produced in the early 1930s for portable valve (tube) radio LT supply (2, 4 or 6 V) by adding silica to the sulfuric acid. By this time the glass case was being replaced by celluloid and later in 1930s other plastics. Earlier "wet" cells in glass jars used special valves to allow tilt from vertical to one horizontal direction in 1927 to 1931 or 1932. The gel cells were less likely to leak when the portable set was handled roughly. A modern gel battery is a VRLA battery with a gelated